Impact of simulated acid rain on chemical properties of Nyalau series soil and its leachate

Greenhouse gases can cause acid rain, which in turn degrades soil chemical properties. This research was conducted to determine the effects of simulated acid rain (SAR) on the chemical properties of Nyalau series (Typic paleudults). A 45-day laboratory leaching and incubation study (control conditions) was conducted following standard procedures include preparing simulated acid rain with specific pH levels, followed by experimental design/plan and systematically analyzing both soil and leachate for chemical changes over the 45-day period. Six treatments five of which were SAR (pH 3.5, 4.0, 4.5, 5.0, and 5.5) and one control referred to as natural rainwater (pH 6.0) were evaluated. From the study, the SAR had significant effects on the chemical properties of the soil and its leachate. The pH of 3.5 of SAR treatments decreased soil pH, K+, and fertility index. In contrast, the contents of Mg2+, Na+, SO42−, NO3−, and acidity were higher at the lower SAR pH. Furthermore, K+ and Mg2+ in the leachate significantly increased with increasing acidity of the SAR. The changes in Ca2+ and NH4+ between the soil and its leachate were positively correlated (r = 0.84 and 0.86), whereas the changes in NO3− negatively correlated (r = − 0.82). The novelty of these results lies in the discovery of significant alterations in soil chemistry due to simulated acid rain (SAR), particularly impacting soil fertility and nutrient availability, with notable positive and negative correlations among specific ions where prolonged exposure to acid rain could negatively affect the moderately tolerant to acidic and nutrient-poor soils. Acid rain can negatively affect soil fertility and the general soils ecosystem functions. Long-term field studies are required to consolidate the findings of this present study in order to reveal the sustained impact of SAR on tropical forest ecosystems, particularly concerning soil health, plant tolerance, and potential shifts in biodiversity and ecological balance.


Leaching experiment design and setup
The experiment was conducted using 18 polyethylene soil columns having 16 cm diameter and 28 cm depth and fitted with 26 holes (3 mm in diameter) at the bottom.The holes evenly distributed in a uniform circular pattern for optimal drainage.Analytical grade tissue paper was placed at the bottom of the column (to prevent soil loss) after which the column was filled with 270 g soil.Soil bulk density 44 at the the undisturbed agricultural field site was first quantified, and the value was used to estimate the quantity of soil (i.e.soil without water content) to be used/ correspond with soil compaction in each column.This resulted in each empty soil column being filled with

Treatment preparation and application
The soil in the columns were exposed to SAR by applying water with pH of 3.5, 4.0, 4.5, 5.0, 5.5, and 6.0.The pH 6.0 served as natural rainwater (control treatment).The selected SAR pH values of 3.5, 4.0, 5.0 and 5.5 were chosen to represent a range of acid deposition scenarios, from extreme to more moderate conditions enabling the study of soil responses under different environmental stress levels.A pH of 3.5 represents the worst-case scenario for acid rain worldwide and indicates the most severe environmental impacts.The other values, 4.0, 5.0 and 5.5, serve as projections ranging from extreme acidity to normal rainwater conditions.This range provides a comprehensive understanding of how different acidity levels can affect ecosystems, making the study relevant to real-world scenarios.
Water with varying pH levels was prepared by adding 0.1 molar H 2 SO 4 and HNO 3 in a 3:2 volume-to-volume ratio to distilled water, after which the pH was adjusted to the desired level 45 .The chemical properties of the SAR are presented in Table 2.Each treatment had three replications; thus, the total experimental units were 18.The experimental units were arranged in a completely randomized design (CRD) with aset up of 6 m × 4 m room having a 76% relative humidity and a temperature of 21 °C.Approximately 318 mL of SAR were applied to each soil column and this volume was based on the field capacity of the soil using a drip system operating at a flow rate of 2.71 mL s −1 .The soil in the leaching columns were exposed to the SAR once every three days for 45 days (15 applications in total) at 8 pm.SAR application interval was based on average monthly/yearly rainfall events in Bintulu (MMD, Unpublished data), Sarawak, Malaysia.At the end of the experiment, the soil and its leachate were collected for chemical analysis.

Analysis of selected chemical properties of Nyalau series
After the incubation experiment, the soil samples in the columns were collected, air-dried, and sieved to pass through a 2 mm sieve for chemical analysis.The soil pH was measured in distilled water at a soil/water ratio of 1:2.5 39 .The CEC in mg/kg of the soil was determined using 1 M ammonium acetate buffered at pH 7. Exchangeable base cations were extracted using 100 mL of 1 M ammonium acetate buffered at pH 7, after which the filtrates were analyzed to determine the concentrations of exchangeable K, Ca, Na, and Mg using Flame Atomic Absorption Spectrometery (AAS) (iCE 300, Thermo Fisher Scientific®, NSW, Australia).The concentration of available P in the soil filtrate was determined using a UV-VIS spectrophotometer (UV-1800, Shimadzu, Kyoto, Japan) operated at 820 nm wavelength after extracting the soils using Bray's solution (0.03 N of ammonium fluoride, NH 4 Fl in 0.025 N of HCI) 40 .
Soil available NO 3 − and NH 4 + were determined using Keeney and Nelson method 41 followed by steam distillation 40 .Soil acidity, Al 3+ , and H + were determined using the titration method 42 .The soil available sulfate was extracted using 0.5 M of NaHCO 3 , after which the extract was analyzed using ion chromatograph IC-MS (AI300, PerkinElmer Inc., USA).The loss-on-ignition (LOI) method was used to determine soil organic matter and total organic carbon 43 .A 5 g of oven-dried sample (dried at 6 °C for 24 h) was weighed into a porcelain dish, placed in a muffle furnace, and heated at 300 °C for 1 h to determine soil organic matter content.
The Soil Fertility Index (SFI; Eq. 1) and Soil Evaluation Factor (SEF; Eq. 2) of Nyalau series were calculated using the formulas of Moran et al. 46 and Lu et al. 47 , respectively.www.nature.com/scientificreports/

Statistical analysis
One-way analysis of variance (ANOVA) was used to detect between-treatment before after which treatment means were compare dusing Duncan's New Multiple Range Test (post-hoc analysis) at p ≤ 0.05.Pearson's correlation analysis was conducted to determine the relationship between the chemical properties of the soil and its leachate.In addition, Pearson's correlation analysis was performed to analyze the response of soil and leachate variables across the pH of SARtreatment.The statistical analysis was performed using SAS version 9.4 48 .
Hierarchical cluster analysis (CA) was performed to find out similar groups of soil properties depending of origin (one soil type) and concentration.The CA was performed on the various chemical properties simulated acid rain and leachate, using a distance cluster between 15 and 20 49,50 .A distance criterion between two variables express how closely correlate within the group.Two cluster analyses by means of hierarchical dendrograms were performed by using SPSS 28.0 (IBM SPSS Statistics, USA) applied to the SAR and soil leachate.All these analysis collectively allowed for interpreting how SAR treatments affected soil and leachate composition, guiding conclusions on acid rain's impact.

Effects of simulated acid rain treatments on soil properties
Soil pH, K + , SFI and SEF significantly decreased with increasing acidity of SAR.As example, significant decrease in soil pH and SFI (2.21% reduction) were recorded when the soil was exposed to SAR with pH 4.0 and pH 3.5.Potassium ions in the soil decreased from 0.037 to 0.019 mg kg −1 (48.64% reduction).Contrastingly, Mg 2+ , Na + , SO 4 2− , NO 3 − , and soil acidity significantly increased with increasing acidity of the SAR.Relative to control (natural rainwater) the soil which was exposed to SAR with a pH of 3.5 increased Mg 2+ , Na + , SO 4 2− , NO 3 − , and acidity by 193.33%, 101.30%, 46.2%, 18.65% and 22.02%, respectively.Furthermore, significant reduction was observed in the level of Al 3+ , H + , and Zn 2+ in soils exposed to SAR with pH 5.0.However, the K + , Ca 2+ and Zn 2+ cations decreased with increasing acidity of SAR (pH 4.0 and below).Similarly, available P in the soil significantly reduced from 1.62 mg kg −1 at SAR of pH 6.0 to 1.43 mg kg −1 at pH of 4.5, whereas SAR with pH 3.5 recorded an available P value of 1.57 mg kg −1 .Furthermore, the Soil CEC, Ca 2+ , Fe 2+ , and NH 4 + fluctuated across the SAR treatments whereas SEF generally remained unchanged (Table 3).

Effects of simulated acid rain (SAR) treatments on leachate properties
There was significant increase in K + and Mg 2+ concentrations in leachate as SAR levels were decreased from 4.0 to 3.5 (Table 4).K + ions increased from 5.62 mg L −1 (SAR at pH 6.0) to 6.65 mg L −1 (SAR at pH 3.5) whereas Mg 2+ ions increased from 0.72 mg L −1 (SAR at pH 6.0) to 0.83 mg L −1 (SAR at pH 3.5).The Na + in the leachate significantly increased from 1.92 to 4.63 mg L −1 with increasing SAR acidity.The continued acidification reduced Na + in the leachate to 2.68 mg L −1 (pH 3.5).The leachate of PO 4 2− concentration did not significantly differences regardless of SAR pH.Other variables fluctuated across the SAR pH (Table 4).

Relationship between soil and leachate properties
The relationship between the soil and its leachate properties was analyzed to determine acid deposition's effect on nutrients leaching or retention by the Nyalau series.The Pearson's correlation analysis revealed that the changes in Ca 2+ and NH 4 + between the soil and its leachate positively correlated and the Pearson's correlation coefficient (r) values were 0.84 and 0.86, respectively.However, the NO 3 − in the soil and its leachate was correlated negatively (r = − 0.82).The correlation for the other variables were not significant (Fig. 2).

Cluster analysis for soil and leachate properties
The findings of CA are presented in two hierarchical dendrograms representing soil (Fig. 3A) and leachate (Fig. 3B).The dendrogram for soil comprise 3 clusters (Fig. 3A).NH 4 + and NO 3 − comprise first cluster and SO 4 2− , SFI, CEC and SEF comprise the second cluster and are associated with a low distance criterion around 1. The rest of the chemical properties acidity, pHwater, pH KCl , H + , Na + , Ca 2+ , Mg 2+ , Al 3+ , Cu 2+ , K + , Zn 2+ and Fe 2+ form (1) SFI = pH + organic matter %, dry soil basis + available P mg kg −1 , dry soil + Exch.K ceq kg −1 , dry soil + Exch.Ca ceq kg −1 , dry soil + Exch.Mg ceq kg −1 , dry soil − Exch.Al ceq kg −1 , dry soil www.nature.com/scientificreports/ the third cluster and they are associated in a very low distance at around 1. In Fig. 3B, the first cluster contains Cu 2+ ,NO 3 − ,PO 4 2− , NO 2 − , NH 4 + , Mg 2+ , Cl − , Salinity and S 2− and they are positioned at a very low distance around 1. In the second cluster, pH, K + , Ca 2+ and Fe 2+ form a group with a distance of CA below 3 whereas electrical conductivity (EC) is placed separately than cluster 1 and 2 with a high distance criteria at 25.

Simulated acid rain and natural rainwater on soil properties of Nyalau series
Generally, the SAR treatments, including control, initially decreased soil pH (4.84).The pH ofsoilwith SAR pH below 4.5 (Table 3) was significantly low and this may cause reduction in the soil fertility index.Additionally, the soil exchangeable Al 3+ and H + were significantly increased because aluminium hydrolysis increases with increasing soil acidity.For example, a complete hydrolysis of one mole Al 3+ ions produces three moles H + ions + in acid rain favour the dominance of H + ions on the soil exchange sites such that soil CEC is disproportionately dominated by hydrogen ions instead of base cations, especially K. Significant leaching of K + in soil with SAR at pH 3.5 was expected due to the high acidity ofthis treatment.The dominance of stronger complementary adsorbed cations at the soil exchange sites could partly explain the loss of K into the leachate 26,51 .Acidic rainwater gradually diminishes exchangeable cations in topsoil because it facilitates changes in the nutrient pool and leaching of nutrients from the soil profile 44 .This observation is supported by Zhang et al. 11 , who reported significantly higher effluent K + concentration of SAR at pH 3 and below.
The low SAR pH were responsible for low variations in Ca 2+ , Mg 2+ , Na + , acidity, NH 4 + , and SO 4 2− in the soils compared with soil treated with natural rainwater.This finding is similar to that of Rampazzo and Blum 52 who reported that exposing parent rock material to acid rain, inspite of having 30-80% calcite, reduced CEC and base saturation, particularly Ca contents.This suggests the fertility and the overall productivity of soils will decline if they are exposed to acid deposition for a long time.A notable reduction in soil pH enhances the solubility of aluminium, consequently elevating the concentration of Al 3+ ions in the leachate.This finding aligns with Mulder et al. 53 observation, where they reported the phytotoxic effects due to increased dissolution of Al 3+ in soil leaching experiments conducted in both the Netherlands and New Hampshire, USA.
Soil Zn 2+ solubility has increase with decreasing pH (3.5-6.0) because the solubility of Zn decreases with increasing soil pH.High levels of soil contamination, with soluble Zn 2+ reaching 19,570 mg/kg and Cu 2+ up to 322.4 mg/kg 38 , enhance the phytoavailability of heavy metals 14 , leading to increased uptake by plants.The very acidic SAR treatments increased soil exchangeable sulfate 46.20% because of sulfate adsorption to form sulfuric acid which upon decomposing, releases H + and SO 4 2− ion.This reaction occurs at low soil pH 54 .Soil available ammonium increase with increasing acidity of SAR.The increase in NH 4 + concentration is consistent with the report of Johnson et al. 55 , who demonstrated that acid rain increases nitrogen mineralization and nitrification in forest soils.

Simulated acid rain and natural rainwater on leachate properties of Nyalau series
Leachate pH was highest with lower SAR pH compared with natural rainwater.According to De Walle et al. 56 , the increase in leachate pH was due to the accumulation of base cations, especially Ca 2+ and Mg 2+ (Table 4).This result also explains the movement of Ca 2+ and Mg 2+ down the soil profile, corroborating the results of Zhang et al. 11 on Latosol of Southern China.Low electrical conductivity and salinity values were recorded with lower SAR pH because the accumulation of base cations in the leachate increased the EC of the soil.The base cations in the leachate of the lower SAR pHs were higher than with the natural rainwater (Table 4).This present study suggests that acid rain causes leaching of the bases and this could cause ground water pollution through enrichment through lost nutrients from the soil profile.

Overall implication of varying simulated acid rain on soil and leachate properties
The incubation of Nyalau soil series with SAR generally had negative effects on pH, K, Fe and NO 3 of the soil and its leachate.This includes a decrease in soil pH, indicating increased acidity, and reductions in the concentrations of potassium (K), iron (Fe), and nitrate (NO 3 ) in the soil.The results indicate that when the pH of SAR decreases from 6.0 to 3.5, the pH and potassium (K) content in the soil and leachate also decrease.This is confirmed by the data in Tables 3 and 4. The increased soil acidity with the low pH SAR is related to high H + concentration.The accumulation of H + from acid deposition increased the soil acidity 27,51 .Increase in the soil acidity through acid deposition might have affected the solubility of heavy metals such as Fe, as observed in the soil with low SAR pH.Furthermore, acidic pollutants can cause P fixation by Al and Fe in soils 57 and this explain low available P content in this present study (Table 3).The positive relationship between soil and leachate for Ca 2+ and NH 4 + was due to insufficient time (45 days) for leaching of cations from the soil.This slower leaching rate is due to the www.nature.com/scientificreports/complex interplay of physical, chemical, and environmental factors within the soil.Essentially, these ions are not as readily mobilized or washed out of the soil compared to other elements, indicating a delayed response to the leaching process influenced by soil composition and conditions 58 .More than that the similarities of SO 4 2− , SFI, CEC and SEF in hierarchical dendrograms of soil have shown that the fertility of Nyalau series soil have also influenced by SO 4 2− .We believe it was happening because of the presence of sulphuric acid (H 2 SO 4 ) from SAR treatments.Our argument is consistent with finding in Table 3 recorded higher SO 4 2− content under low SAR pH treatments.Similar study reported by Hüttl and Frielinghaus 59 in Eastern, Germany who shows that air pollutant or acid rain content with H 2 SO 4 could reduce the soil fertility accelerating soil acidification.In the leachate hierarchical dendrograms, there are similarities of soil water pH, K and Ca.This results reliable comes from accumulation of base cation while exposure to SAR as discussing in previous section.

Management implication of simulated acid rain on soil and leachate properties
Even with a short incubation study (45 days), we found a 2.21% reduction in the fertility of Nyalau series and 5.43% reduction in soil acidity as compared when exposed to natural rainwater (control treatment).The lower SFI of the soil in the present study (11.94) compared with research on a secondary forest in Lundu, Sarawak, by Perumal et al. 60 where SFI of 19.63 was recorded, indicates the prolonged negative impact of acid rain on soil fertility.These results showed that acid rain impacted soil and leachate properties, and it is possible that prolonged acid rain exposure will further modify soils of the Nyalau series detrimentally.
Therefore, for a comprehensive understanding of acid rain's effects, a long term study, possibly over a year, is recommended.This allows for observing long-term ecological and soil changes.Complementing this with advanced modeling would provide a holistic view, predicting future impacts and aiding in effective environmental management strategies, crucial for sustaining ecosystems and agricultural productivity in the face of environmental changes.Therefore, understanding the prolonged impacts of acid rain on soil properties is not only an ecological necessity but also crucial for human sustainability.

Conclusion
The study focused on the impact of simulated acid rain (SAR) on the Nyalau series soil, examining a range of acidity levels from less acidic (pH 5.5 and 5.0) to more acidic (pH 4.0 and 3.5).It was found that with increasing acidity, especially at pH 3.5, the soil experienced significant changes: a decrease in pH, potassium, and fertility, and an increase in magnesium, sodium, sulfate, nitrate, and overall acidity.The leachate from the soil also showed increased levels of potassium and magnesium, indicating a leaching effect that could lead to nutrient deficiencies for plants.The study also noted a positive correlation between changes in calcium and ammonium levels in both soil and leachate, and a negative correlation in nitrate levels, highlighting complex interactions between soil acidity and nutrient dynamics.
The results of our study have important practical implications for both land management and environmental policy.Land managers are suggested to regularly conduct comprehensive soil health assessments, especially in areas vulnerable to acid rain or soil acidification.These assessments should go beyond simply measuring pH and consider chemical properties such as K + , Mg 2+ and NO 3 − to inform soil treatment plans.In terms of policy, the observed deleterious effects of acidic treatments on soil properties call for stricter pollution regulations to curb acid rain, and the data could further guide the establishment of safe areas for agriculture and forestry based on the resilience of soils to acidification.

Figure 1 .
Figure 1.Location of the soil sampling sites in Universiti Putra Malaysia, Bintulu, Sarawak.Sampling were conducted ramdomly from several points in study sites.

Figure 2 .
Figure 2. Trends of selected soil and leachate properties of Nyalau series (Typic paleudults) soil after exposure to simulated acid rain.Correlation analysis was conducted, and the relationship was indicated by the Pearson's correlation coefficient (r) and probability level significant at p ≤ 0.05.

Figure 3 .
Figure 3. Hierarchical dendrogram for chemicals properties found in soil (A) and leachate (B) using Ward's method.

Table 2 .
Chemical properties of simulated acid rain used in the incubation study.

Table 4 .
Leachate pH, and chemical properties after exposure to simulated acid rain.Values were expressed as means ± standard error (n = 3).Different letters within the same row indicate significantly different means at p ≤ 0.05 using a Duncan test.
a Vol.:(0123456789) Scientific Reports | (2024) 14:3534 | https://doi.org/10.1038/s41598-024-52758-1www.nature.com/scientificreports/ to further decrease soil pH and this chemical reaction reduces soil CEC.This finding corroborates that of Zhang et al. 11 who explained that acid rain increases soil acidity and H + ions, leading to loss of mineral structure.Loss in mineral structure has been implicated in soil fertility decline.Wei et al. 51 also reported that acid rain reduces soil fertility because it reduces soil pH and cation retention capacity.Although soils have strong pH buffering capacity, the SO 4 2− , H + , NO 3 − , and NH 4